High in specific strength and specific modulus, carbon fiber reinforced composite materials containing carbon fibers as reinforcing fibers have been used in recent years in manufacturing structural materials for aircraft and automobiles, sporting goods such as tennis rackets, golf shafts, and fishing rods, as well as general industrial applications.
Carbon fiber reinforced composite material can be manufactured by, for example, curing plates of prepreg, that is, a sheet-like intermediate prepared by impregnating reinforcing fiber with uncured matrix resin or curing an intermediate prepared by injecting liquid resin into reinforcing fiber placed in a mold, which is called the resin transfer molding method. Of these production methods, a common prepreg based process includes stacking a plurality of prepreg sheets and then heat and press them to provide carbon fiber reinforced composite material. From the viewpoint of productivity related characteristics including processability, thermosetting resin such as epoxy resin is used frequently as matrix resin for prepreg.
In field of structural materials for aircraft and automobiles, in particular, rigorous characteristics requirements, such as for high tensile strength and compressive strength, have been imposed on carbon fiber reinforced composite materials in recent years as they have come in wider use. However, trade-off relation often exists between tensile strength and compression strength of carbon fiber reinforced composite material, making it very difficult to realize high level of tensile strength and compression strength at the same time.
Effective methods for improving the tensile strength of carbon fiber reinforced composite materials include using reinforcement fiber with higher strength and increasing the volume fraction (Vf) of fiber. Many methods have been proposed so far with the aim of obtaining high-strength reinforcement fibers (see Patent document 1). In general, as the strength of reinforcement fiber is increased, it tends to become more difficult to make effective use of the inherent strength of the fiber, but this proposal contains no statement on strength enhancement that can be achieved by using carbon fiber for reinforcement. It is also known that even if reinforcement fibers with the same level of strength are used, reinforced materials may vary widely in tensile strength utilization rate depending on the matrix resin combined and the conditions used for molding. A large residual thermal stress distortion tends to be left in carbon fiber reinforced composite material, preventing the development of high strength, when it is cured at a temperature above 180° C. Therefore, even if high-strength carbon fibers are obtained, there still remain technical problems to be solved to produce carbon fiber reinforced composite material that maintain the high strength.
It has been shown that a high tensile strength utilization rate can be developed if the tensile rupture elongation and fracture toughness KIC of matrix resin is in a specific relationship (see Patent document 2). However, if a large amount of thermoplastic resin or rubber components are added to the matrix resin with the aim of increasing the fracture toughness KIC, the viscosity will increase to cause a deterioration in the processability and handleability in the prepreg production process.
Furthermore, compression strength is also an important characteristic of carbon fiber reinforced composite material when it is used as structural member. The resin compositions disclosed so far as a component for producing carbon fiber reinforced composite material with high compression strength include epoxy resin compositions containing tetraglycidyl diaminodiphenyl methane, bifunctional epoxy resin (such as bisphenol A type epoxy resin and diglycidyl resorcinol), and 3,3′-diaminodiphenyl sulfone (see Patent document 3), epoxy resin compositions containing polyfunctional epoxy resin, a diglycidyl aniline derivative, and 4,4′-diaminodiphenyl sulfone (see Patent document 4), and epoxy resin compositions containing polyfunctional epoxy resin, epoxy resin with a special backbone, and 3,3′-diaminodiphenyl sulfone (see Patent document 5). Although they serve to increase the compression strength, it is still difficult to maintain both compression strength and tensile strength at a high level.